On 4/13/2018 5:11 PM, Michal Hocko wrote:
On Fri 13-04-18 16:57:06, Chintan Pandya wrote:
On 4/13/2018 4:39 PM, Michal Hocko wrote:
On Fri 13-04-18 16:15:26, Chintan Pandya wrote:
On 4/13/2018 4:10 PM, Anshuman Khandual wrote:
On 04/13/2018 03:47 PM, Chintan Pandya wrote:
On 4/13/2018 3:29 PM, Anshuman Khandual wrote:
On 04/13/2018 02:46 PM, Chintan Pandya wrote:
Unmap legs do call vunmap_page_range() irrespective of
debug_pagealloc_enabled() is enabled or not. So, remove
redundant check and optional vunmap_page_range() routines.
vunmap_page_range() tears down the page table entries and does
not really flush related TLB entries normally unless page alloc
debug is enabled where it wants to make sure no stale mapping is
still around for debug purpose. Deferring TLB flush improves
performance. This patch will force TLB flush during each page
table tear down and hence not desirable.
Deferred TLB invalidation will surely improve performance. But force
flush can help in detecting invalid access right then and there. I
Deferred TLB invalidation was a choice made some time ago with the
commit db64fe02258f1507e ("mm: rewrite vmap layer") as these vmalloc
mappings wont be used other than inside the kernel and TLB gets
flushed when they are reused. This way it can still avail the benefit
of deferred TLB flushing without exposing itself to invalid accesses.
chose later. May be I should have clean up the vmap tear down code
as well where it actually does the TLB invalidation.
Or make TLB invalidation in free_unmap_vmap_area() be dependent upon
debug_pagealloc_enabled().
Immediate TLB invalidation needs to be dependent on debug_pagealloc_
enabled() and should be done only for debug purpose. Contrary to that
is not desirable.
Okay. I will raise v2 for that.
More importantly. Your changelog absolutely lacks the _why_ part. It
just states what the code does which is not all that hard to read from
the diff. It is usually much more important to present _why_ the patch
is an improvement and worth merging.
It is improving performance in debug scenario.
Do not forget to add some numbers presenting the benefits when
resubmitting.
Okay.
More than that, I see it
as a clean up. Sure, I will try to address *why* in next change log. >
As Anshuman pointed out the current code layout is deliberate. If you
believe that reasons mentioned previously are not valid then dispute
them and provide your arguments in the changelog.
Here, the trade off is, performance vs catching use-after-free. Original
code is preferring performance gains. At first, it seemed to me that
stability is more important than performance. But giving more thoughts
on this (and reading commit db64fe02258f1507e ("mm: rewrite vmap
layer")), I feel that use-after-free is client side wrong-doing. vmap
layer need not loose its best case settings for potential client side
mistakes. For that, vmap layer can provide debug settings. So, I plan
to do TLB flush conditional on debug settings.
Chintan
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